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The ClientTagBasedModelTypeProcessor is a crucial piece of the USS codepath. It lives on the model thread and performs the tracking of sync metadata for the ModelTypeSyncBridge that owns it by implementing the ModelTypeChangeProcessor interface, as well as sending commit requests to the ModelTypeWorker on the sync thread via the CommitQueue interface and receiving updates from the same worker via the ModelTypeProcessor interface.

This processor supports types that use a client tag, which is currently includes all except bookmarks. This means all changes in flight (either incoming remote changes provided via the ModelTypeWorker, or local changes reported by the ModelTypeSyncBridge) must specify a client tag, which is considered (after being hashed) the main global identifier of a sync entity.


The bridge owns a processor object at all times and operates on the same thread as it. If sync is disabled, the processor is destroyed but a new one is immediately created to replace it.

Processor State Machines

The processor sits between the model bridge and the sync engine. It has knowledge of what state each is in based on the calls it has received and performed. The states are not stored explicitly, but are implicit based on state stored in the processor. Here are the states of each, with notes on their transitions and how to determine them.

Model States

Sync States

Processor States

Based on the interplay of the model and sync states, the processor effectively progresses through 3 states worth noting:

Entity Tracker

The ProcessorEntity tracks the state of individual entities for the processor. It keeps the EntityMetadata proto in memory, as well as any pending commit data until it gets acked by the server. It also stores the special commit_requested_sequence_number_, which tracks the sequence number of the last version that's been sent to the server.

The tracker holds the metadata in memory forever, which is needed so we know what to update the on-disk memory with when we get a new local or remote change. Changing this would require being able to handle updates asynchronously.